Is what a company gives to a user in exchange for 1000 customers or $0.1 for each download considered an unknown recompense (ja'l majhul)? And what are the criteria based on which recompense is deemed unknown?

If the files to be uploaded and downloaded are permissible, then there is no objection to al-ja'alah (a reward for a specific task) for downloading them, provided that the ju'l (the reward) is known in kind and quantity, according to the majority of scholars. Some scholars hold that ambiguity does not prevent the validity of al-ja'alah if it does not prevent delivery. What you mentioned, that the company gives one dollar for every thousand customers, is not unknown, and this is permissible. However, if the ambiguity prevents delivery, such as if the company says: "Whoever brings me a thousand customers shall have a reward," without specifying it, then al-ja'alah is not valid. If the number of customers does not reach the agreed-upon limit, some scholars believe that no reward is due, while others say that a reward proportionate to the work done is due if benefit was derived from it.